Circular Polarization Selective Surfaces (CPSS) of the present invention have similar applications to those known in the art for vertical and horizontal Linear Polarization Selective Surfaces (LPSS). The surfaces according to the present invention may be used in a wide range of reflector antennas such as in the reduction of aperture blockage by the sub-reflector of a symmetrical dual-reflector antenna, a dual-reflector antenna with a CPSS sub-reflector in which both right and left polarizations can be used at the same frequency with a separate feed network for each frequency.
Several configurations of circular polarization select surfaces are presently known in the art. These known configurations have serious disadvantages for some particular applications compared to a configuration according to the present invention.
A first known configuration is based on optics and consists of three superimposed plates. The three superimposed plates are, in order, a quarter-wave plate that changes circular polarization to linear polarization, a linear polarization selective surf ace and another quarter-wave plate that changes linear polarization into circular polarization. This type of configuration is only actually suitable for short wavelengths, such as millimeter waves, since the three plates become rather bulky for longer wavelengths.
A second known configuration uses two planar arrays wherein the first array receives the incoming signal and passes it to an array of networks. The networks discriminate between one polarization and the other and either reflects the signal back or passes it to the other array which will transmit that signal. This is a very complex design due to the large number of networks required and their physical size.
A third known configuration consists of a planar array of crossed dipoles connected by half-wavelength transmission lines the vertical dipoles in the array of crossed dipoles being separated from the horizontal dipoles by a quarter-wavelength. This type of array is disclosed in Canadian Patent Application 546,499 entitled "Polarization Selective Surface For Circular Polarization" which is assigned to Her Majesty the Queen in Right of Canada. However, the transmission lines with that configuration are difficult to make at frequencies over 1 GHz since they are very small and a practical design needs thousand of transmission lines.
A fourth known configuration consists of a planar array of a multitude of resonating elements arranged in a prescribed pattern on and in a dielectric slab. Each resonating element can be made from a straight wire which is one wavelength in length with two end sections of the wire bent at ninety degrees from the central section and from each other. The central sections are arranged parallel in the array with each end section on opposite surfaces of the array being arranged in the same direction. This type of design is described in French Patent No. 1,512,598.
The third and fourth described configurations for circular polarization selective surfaces uses dielectric slabs for mechanical support. The dielectric material, however, causes unwanted reflections of the incoming waves and also generates surface waves that degrade the performance of the array. The only practical way of reducing those reflections and surface waves is to use a dielectric of low permittivity such as Styrofoam. However, low permittivity dielectrics like Styrofoam are quite soft and cannot be precisely machined. Furthermore, they are readily deformed which makes them unsuitable as supports for these arrays.